Prior art computer aided design (CAD) software is known to include complementary tool suites for designing and analyzing the package of a die, e.g., a microprocessor. A “package” is the physical interconnection between the die and, for example, a printed circuit board (PCB). A typical package has several interconnected layers between its top layer (L1), which connects to the die, and its bottom layer (L2), which connects to the PCB.
A package “design” is a hierarchical and symbolic digital model of the package circuit. Those skilled in the art appreciate that hardware description languages (HDLs) may be used to formulate this digital model. The digital model consists of linked design elements that simulate the package circuit. The design elements are for example digital representations of the transistors, resistors, logic gates, traces (i.e., intra-layer conductors), capacitors, vias (i.e., inter-layer connectors), and wire bonds that make up the simulated schematic circuit.
The design elements and interconnections are collated and defined in a design database, which is a textual representation of the package design. The design database may further describe the package design in terms of higher-layer cells consisting of two or more design elements, and the connections between cells. Each “net” in the package design describes the linked conductors (e.g., traces of a layer and vias between layers) that form a circuit between an input and an output of the package. The CAD software may automatically route traces within a given layer of the package design; it may further automatically route vias between layers of the package design.
The design database is processed by the CAD software to perform circuit simulation. The CAD software is for example used to model a signal through the package and over a net (i.e., a “signal net”). Substrate laminate technologies and bond interconnections may also be evaluated through the CAD software.
One exemplary prior art CAD software is Advanced Package Designer (APD) from Cadence Design Systems, Inc., of San Jose, Calif. Such CAD software is known to include verification procedures and dynamic feedback that evaluate design accuracy against a set of physical and electrical design rules, or constraints. Physical design constraints help to ensure manufacturability; electrical design constraints help to ensure electrical specifications of the design. By way of example, this CAD software generates a Design Rule Check (DRC) indicating whether the design meets the various constraints. The prior art CAD software also provides a graphical user interface to view all or part of the package design in two dimensions, for example in a flat or perspective rendition, or with layers overlaid relative to one another.
FIG. 1 illustrates one prior art system 10 for designing a package with prior art CAD software 12. CAD software 12 is stored within a computer 14, initially within a storage unit 16. A processor 18 of computer 14 operates CAD software 12 in response to user inputs at an input interface 20 (e.g., a computer keyboard and mouse). As those skilled in the art appreciate, when initialized, CAD software 12 may also load into internal memory 22 of computer 14. A human designer at input interface 20 then controls CAD software 12, through processor 18, to create a package design 24, also stored within memory 22. The designer can command processor 18 and CAD software 12 to graphically show package design 24 at a graphical user interface 26 (e.g., a computer monitor) of system 10. Illustratively, package design 24 is graphically depicted on a display 28 of graphical user interface 26 as a five-layer graphical model 24A shown in FIG. 2.
FIG. 2 illustrates detail of graphical model 24A. L1 of model 24A couples with a die, and L2 of model 24A couples with a PCB. Layers I(1), I(2) and I(3) of model 24A represent intermediate layers of package design 24. Layers L1, I(1), I(2), I(3), L2 are shown as distinct elements and without proper or to-scale orientations for ease of illustration. An illustrative signal net 30 is shown from an input connector 32 to an output connector 34 of model 24A. Signal net 30 traverses design elements in the form of traces and vias between connectors 32, 34: via 35 from connector 32 of L1 to trace 36 of I(1); trace 36 within I(1) from via 35 to via 38; via 38 from trace 36 of I(1) to trace 40 of I(2); trace 40 within I(2) from via 38 to via 42 from trace 40 of I(2) to trace 44 of I(3); trace 44 within I(3) from via 46, which terminates at connector 34 of L2.
Design 24 also has power vias 60 and ground vias 70 connected, respectively, to L2 power pads 62 and L2 ground 72. As known to those skilled in the art, power vias 60 are designed to provide power (i.e., a “power net” ) to layers L1, I(1)-I(3), L2 of design 24, for use by design elements and signals of these respective layers. Ground vias 70 similarly provide grounding (i.e., a “ground net”) to layers L1, I(1)-I(3), L2 of design 24, for use by design elements and signals of these respective layers. Those skilled in the art appreciate that a typical design 24 can and usually does have many more power and ground vias 60, 70 (and respective power and ground pads 62, 72) than what is shown in FIG. 2; though only a few power and ground vias 60, 70 (and respective power and ground pads 62, 72) are shown for purposes of illustration.
With further regard to FIG. 1, CAD software 12 is also operable to generate a design database 50. In one example, design database 50 textually defines signal net 30 of FIG. 2: signal net 30 is defined by connectors 32, 24, traces 36, 40, 44, and vias 35, 38, 42, 46. Design database 50 also defines power and ground vias 60, 70, and L2 pads 62, 72. Design database 50 further includes parameters (often called a “netlist”) to ensure that signal net 30 has start and end points (i.e., connectors 32, 34 for signal net 30). The netlist also typically defines physical size dimensions of package design 24. A designer can manipulate design database 50 to develop the desired package design 24.
CAD software 12 utilizes design rules 52 to generate one or more DRCs 54 in the event that a design element or signal net of package design 24 exceeds a manufacturing constraint or electrical specification. By way of example, design rules 52 may specify that a trace width of trace 36 is 20 μm, to ensure manufacturability. If a designer of system 10 implements trace 36 with 10 μm, for example, then CAD software 12 generates a DRC 54A, which may be graphically displayed on model 24A, as shown in FIG. 2. The user is thus made aware that a problem may exist with trace 36.
Those skilled in the art appreciate that package design 24 often has more than the five layers illustrated in model 24A; however only five layers are shown in FIG. 2 for ease of illustration. For example, it is common that package design 24 include ground layers between each layer with signal traces I(1), I(2) and I(3); however these ground layers are not shown to simplify illustration. Those skilled in the art also appreciate that package design 24 also typically has many more signal nets and other design elements than illustrated signal net 30.
FIG. 3 illustrates package model 24A in a side view. FIG. 3 further illustrates how package design 24 connects between a die 80 and a PCB 82. Connector 32 is for example a pad that connects with a solder ball 84 of die 80; connector 34 is for example a pad that connects with signal wires of PCB 82. Similarly, power vias 60 connect with PCB 82 through power pad 62, while ground vias 70 connect with PCB 82 through ground pad 72.
The increased complexity of the modem die has correspondingly increased the complexity of the package design. An example of a complex die includes a Precision Architecture-Reduced Instruction Set Computer (PA-RISC) processor produced by Hewlett Packard Corporation, which has over one billion components. The package for the PA-RISC processor must maintain high signal integrity through its signal nets; however the prior art CAD software does not evaluate the inductance and/or power goals (e.g., safety and/or DC voltage drop goals) of the package as desired by the corresponding die. Accordingly, the package may be physically manufactured, at great expense, before the designer learns that the package is not suitable for operation with the die. By way of example, while the DRCs generated by the prior art CAD software may assist in manufacturability, they do not warn the designer of power and ground delivery problems between the die and the package. Specifically, prior art CAD software 12 does not evaluate the number of power vias 70 and ground vias 70, per pad, of package model 24A. An appropriate number of these power and ground vias 60, 70, per pad, are nonetheless preferred to provide proper signal integrity, voltage differentials, safety and inductances within the package.